1. Field of the Invention
This invention relates to a charge transfer device having MIM structures each formed by sandwiching a tunnel switching film between metal films as a basic unit for charge storage and to a method for driving the charge transfer device.
2. Description of the Related Art
As a typical charge transfer device, a charge coupled device (CCD) is known. The CCD has a basic function of sequentially transferring charges stored in a potential well to a next stage potential well by controlling the depth of the potential wells.
Such a CCD has an element structure in which boron implanted layers are formed by diffusion in a p-Si substrate to form a potential barrier and electrodes for forming potential wells are formed on an SiO.sub.2 film for forming a capacitor, for example. The CCD having the above element structure functions to transfer charges in a lateral direction with respect to the substrate when receiving 2-phase clock signals having a potential difference at the electrodes. The CCD has received much attention as a high density memory which can be formed with a density as high as or higher than a DRAM.
However, when such an element structure is provided on a normal semiconductor Si substrate or amorphous Si substrate, it is generally necessary to effect a high-temperature process at or higher than 500.degree. C. Therefore, formation of the multi-layer structure may provide difficult problems and it is generally considered possible to obtain a laminated structure of two or three layers at most.
The inventors of this invention proposed a charge transfer device of an element structure having a laminated structure of alternately laminated tunnel switching films and metal films and having capacitors connected to the metal films as is disclosed in a U.S. patent application Ser. No. 07/398,271, now U.S. Pat. No. 4,972,370, for example. The charge transfer device has a multi-layered basic structure which is an MIM structure formed by alternately laminating the tunnel switching films (I) formed of organic material and metal films (M) and metal electrodes formed on the top and bottom of the structure. Further, capacitors are connected to the metal films.
With the charge transfer device of the above structure, an input voltage higher than a preset threshold voltage V.sub.th is applied to the metal electrode on the top portion to store charges on the first capacitor. Then, the charges stored on the first capacitor are sequentially transferred in the thickness (depth) direction of the structure by applying a plurality of transfer pulses having a preset phase difference to the MIM structure via the capacitors connected to the respective metal films. That is, the operation of transferring the charges utilizes a 2- or 3-phase driving system using two or three charge storing sections for storing and transferring each information charge.
According to this charge transfer system, a plurality of information charges can be transferred at the same time, but it is necessary to use a large number of charge storing sections and increase the number of necessary layers since two or three charge storing sections are used for storing and transferring each information charge. Therefore, it is extremely disadvantageous in a case where the information charges are stored at a high density.